From your instructions, you want to add the value '5' to each character within your string. To decrypt, you'd simply do a -=5 in lieu of the +=5. You're encoding each character within the string, at least that's how I'm reading the problem. As example, a file such as;
I like apples.
I like pears.
I don't like carrots.
would encrypt to;
std::cout << "Opening File....\n";
while(std::getline(dataFile, s, '\n'))
for(int x = 0; x<s.length(); x++)
std::cout<< "we're done.";